Foundations powerpoint

Foundations and Soil types
Main types of foundation
 Foundations or ‘footings’ come under two
main categories:
 Shallow foundations or ‘spread footings’
include pads, strip footings and rafts.
Shallow foundations
 The guidelines for a shallow foundation
are :
 They are founded near to the finished
ground surface.
 The depth is generally less than the width
of the footing and less than 3m.
 They are used when the surface soils are
strong enough to support the load
imposed upon it.
Deep foundations
 Shallow foundations are unsuitable in
weak or highly compressible soils.
 DEEP foundations include piles, pile walls
and piers.
 Deep foundations are usually at depths
deeper than 3m.
 Deep foundations are used to transmit the
loading to a deeper, more competent
Strip foundations
These are use to
support a line of loads
such as a load
bearing wall.
They could also be
used where the line of
column positions are
so close that
individual pad
foundations would be
Raft Foundations
 These are used to
spread the load from
a structure over a
large area.
 This would normally
be the entire area of
the structure.
Raft foundations
 Raft foundations are
often needed on soft
or loose soils which
have a low load
bearing capacity.
Pads and pile foundations
 Pad foundations support
an individual point of
 Piles are used to support
buildings in poor soil
 A basic pile foundation is
a series of stilts which
rest on a solid load
bearing layer.
 The make up of the soil has a major
influence on the choice of foundation.
 A good soil type needs to be able to cope
with loadings.
 A good soil needs to drain water well.
 A poor soil type will shrink, swell or move
depending on the loads or conditions
placed upon it.
 Silty soil is found in flood plains or around
 Silt holds water well and is soft when wet.
 Silty soil is not a very good foundation
material unless it has been compressed
and hardened, or has been dried out.
 Sand is usually considered favourable
from the standpoint of foundation support.
 It can be a problem though usually due to
 Water raising through a sand deposit can
create an unstable condition.
 Sandy soils can hold water.
 Clay is composed rock particles ground
extremely fine or reduced by weathering.
 Clay soils normally contain water.
 Clay drains slowly and compresses when
foundations are placed upon them.
 Clay has a tendency to absorb water and
 Gravel can be well compacted and allows
water to drain freely.
 Gravel soils do not hold water.
 The variety in particle sizes in gravel
means that even when closely packed it
still contains voids and drains well.
 Gravel is least likely to be affected by
drying out.
Forming foundations
 Profile boards.
 Profiles are used to set
out foundations.
 These are established
outside the limit of the of
the excavation and
always in pairs.
 These can be use as
‘sight rails’ with the aid of
a ‘traveller’ to check on
excavation depth.
Profile boards
 Profile boards are
used by the
groundworker to
establish foundation
 Bricklayers will use
them when they start
to lay the damp proof
course of engineering
Using profile boards.
 A number of nails are
knocked into the top of
the profile directly upon
the relevant line to which
it relates.
 The groundworker will
use the centre line and
the foundation limits to
guide the line of the
excavation along with the
‘traveller’ board to check
the depth.
Using profile boards.
 The bricklayers rely on
the Building Line nails to
guide their setting out.
 Lines will be set using the
nails to indicate the face
of the brickwork.
 From this line the position
of the brickwork can be
marked on the foundation
using a spirit level off the
Engineering bricks
 Engineering bricks
are used below
ground and are strong
and impervious to
 They are used where
the strength of the
brick and the need for
a low level of water
absorption is
Engineering bricks
 Engineering bricks are not susceptible to frost.
 They are load bearing and have a high
compression strength.
They are used up to d.p.c where they will be
exposed to wet and freezing conditions.
They are used below ground because they are
strong and impervious to water.
They are usually used where appearance is not
a factor.
Engineering bricks form a barrier against the
movement of moisture.
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